In the operation of trucks, particularly those which at times may carry lighter loads and at other times may carry much heavier loads, the desirability of additional wheels to support additional loads has been evident for a long time. In the case of farm trucks, for instance, the lighter loads carried during normal farm work, but at other times the much heavier loads carried, such as during the harvesting period of sugar beets, has occasioned the application of tag wheels or supplementary wheels to trucks. In view of the fact that the harvesting time of sugar beets, for instance, is relatively short and it is desirable to transport the sugar beets from the field to the sugar factory or point of rail shipment as soon as possible, in order to reduce loss of sugar content through aging, it is common practice to add to the height of the sides of the truck body when carrying sugar beets. Such increased load, due to the additional weight, may cause the springs or associated parts of the normal or drive wheels to be overloaded with a consequent danger of failure. Thus, it has been common practice to add one or more sets of tag wheels or non-driven wheels ahead of or behind the driven wheels, in order to support the extra load. These tag wheels normally have their own set of springs and are connected by an axle, so that an axle type air brake can be utilized in providing a braking effect corresponding to the heavier load being carried. It is uneconomical to install the tag wheels when use thereof is desired, and to remove the tag wheels after the need for their use has terminated. Also, after the tag wheels have been installed on the truck, there are many times when the load carried by the truck is sufficiently light that the tag wheels are not necessary and it is thus desirable to lift the tag wheels. However, when the driven wheels sink into a mud hole or the like, the non-driven tag wheels may prevent sufficient weight on the driven wheels to provide adequate traction to pull the truck out of the mud hole, with the attendant necessity for a towing truck or tractor to be used. Since a truck mired in, or about to be mired in, a mud hole is proceeding at a very low speed, there is not such a weight upon the driven wheels and springs, even when the truck body contains a very heavy load, that the danger of breaking the springs or other parts is acute. Thus, when a heavily loaded truck has or tends to become mired in a mud hole, it is desirable to lift the tag wheels temporarily, in order to transfer the load to the driven wheels and thereby enable the driven wheels to produce sufficient traction.
My prior U.S. Pat. No. 3,659,671 discloses a tag wheel lift which is suitable for many installations but does involve certain installation expenses which might be avoided, including the remounting of a spring support at each side for the drive wheels and the installation of a hydraulic cylinder and a pivot shaft on the underside of the truck body, between the chassis channels. Some possible purchasers have indicated objection to the shifting of the drive wheel springs for normal operation at light loads, although this shift is a valuable feature for loading of the drive wheels, as when pulling out of a mud hold or the like. An increase in the distance which the tag wheels are lifted, without affecting the drive wheels, appeals to certain possible purchasers.